1. Field of the Invention
The present invention relates to laser diodes and arrays of laser diodes, and further relates to an apparatus for cooling laser diodes during operation. More specifically, the present invention relates to a laser diode array that is actively cooled and provides a laser output having a high average intensity.
2. Description of Related Art
Laser diodes have many advantages over conventional lasers. Laser diodes are small and compact, they are efficient at converting electrical energy into laser energy, and they are reliable. However, when a laser diode is operated at a high average power, it generates a substantial amount of heat in a small volume, thereby raising the temperature of the diode which causes negative effects such as a wavelength shift and a loss of efficiency. If the temperature gets high enough, destruction of the diode package may result. Therefore, present uses of laser diodes are generally limited to applications requiring low average power.
Laser diodes have some similarities to Light Emitting Diodes (LEDs). A typical laser diode is comprised of a semiconductor material, such as Gallium Arsenide (GaAs), that is manufactured to have a pn junction. Like laser diodes, LEDs have a pn junction formed in the semiconductor material. Briefly, the electroluminescence at a pn junction is the result of electrical current applied across the pn junction, associated with the band properties of semiconductor material. As a result of these band properties, an electron may combine with a hole (a lack of an electron) in a recombination that produces radiation. In a typical LED, the effect of an increase in current is to increase the radiative recombination rate well above that of the non-radiative recombination rate. Laser diodes have an additional feature over the LED--facets (reflective surfaces) on each end of pn junction. These facets define a laser cavity, which causes laser oscillation to occur along the length of the pn junction.
The laser diode is a well-known source of laser radiation. The word "laser" is actually an acronym for "Light Amplification by Stimulated Emission of Radiation". The light produced from a laser has many known applications, and it is reasonable to expect that many applications of the laser have yet to be discovered. Laser radiation has application in a wide variety of disciplines, such as communications, medicine, the military, research, and any other field where directed electromagnetic radiation is an advantage.
When compared with other lasers, the laser diode is distinguishable by several features. One distinguishing feature is the size of the laser diode. Laser diodes can be manufactured in a package much smaller than other laser devices such as gas lasers that require larger gas tubes and specialized optics equipment such as Brewster windows, mirrors, spatial filters, and lenses. Another distinguishing feature of the laser diode is its efficiency at converting the input electrical power to output laser intensity. Laser diodes can readily achieve efficiencies of 50% or more in converting electrical energy to laser energy, while other lasers have efficiencies from 10% to less than 1%. For example, the highest efficiency achieved by other lasers is attained by the CO.sub.2 laser, which may attain an efficiency of 10%. Despite the advantage of high efficiency, laser diodes have not been applied in high power applications due primarily to the problem of heat dissipation. Other lasers, such as the copper vapor lasers currently in use for high power applications, have an efficiency of 1% or less. Additional distinguishing features of the laser diode include a fast response to control signals, and simplicity of design. Manufacturing of laser diodes is known in the art, and a capability exists to manufacture many types of laser diodes.
One type of laser diode is the edge emitting laser diodes, often termed "laser diode bars". These diodes emit laser light along a length of their edge. For example, an edge emitting laser diode can output a beam that has an emitting edge length of one centimeter, and a width of 0.3 mm. Typically, an edge emitting laser diode will be manufactured of a single block of GaAs, with a pn junction formed in a plane throughout the block, and the facets positioned on opposing edges of the plane defined by the pn junction. Conductors are constructed on each side of the pn junction so that when current is applied, current passes through the pn junction. The current creates a population inversion across the pn junction, and lasing action can occur.
For any laser diode, heat production is directly associated with the output intensity. Further, a high output intensity results from a large current applied to the diode. The basic mechanisms leading to heat production in a diode are the series resistances of the diode and non-radiative recombination. The series resistances include the resistance of the semiconductor material, and the resistance of the contacts, which produce heat during current flow. The resistances produce heat as current is applied, in an amount of heat flux proportional to I.sup.2 R.
Due to this heat production, a basic limitation on the output intensity of a diode is temperature buildup from heat produced in the pumping process. For maximum efficiency, a diode must have a temperature that is below 25.degree. C. For reliable, long lived operation of the diode, temperatures may be less than 50.degree. C. without substantial loss of efficiency. Temperatures even moderately above 50.degree. C. will substantially affect efficiency and reliability, substantially shortening the useful life of the diode. Furthermore, at higher temperatures the output light will be shifted in wavelength. High temperatures encourage the growth of defects in the laser diode, which decrease efficiency. A larger current may be applied to compensate for the decreased efficiency, which then produces even more heat, encouraging the growth of even more defects and a greater loss of efficiency. If a diode could be maintained at or near its optimum temperature, then the diode will have its maximum efficiency and lifetime, and emit a consistent wavelength.
To reduce the temperature of the diode to an acceptable level while providing a high average output power, diodes are often operated in a pulsed mode wherein current is applied to the diode during only a portion of the operating time. In this mode, the heat has an opportunity to dissipate during the time when current is off. In the pulsed mode of operation, a figure that describes the percentage of time that the diode is pulsed is the "duty cycle". For example, a duty cycle of 1% corresponds to a diode that is actuated with current only once in 100 cycles. Typically, laser diodes will be operated at a duty cycle of 1% and a supplied current of fifty amps/cm of length. However, if some extra cooling is available, higher duty cycles can be attained. If much more substantial cooling were available, continuous (cw) operation may be obtainable for optimum current levels. The cooling problem is of particular significance for arrays of laser diodes.
Arrays of laser diodes include a number of laser diodes positioned closely together. Laser diode arrays may be manufactured in various architectures such as the stacked architecture and the monolithic surface emitting architecture. In one configuration (the "rack and stack" configuration) the laser diodes are positioned in a stacked configuration, one on top of the other. In another configuration, the monolithic surface emitting architecture, a number of edge emitting laser bars are positioned on the surface of a thermally conductive material, next to reflectors angled at 45.degree.. Thus, the laser radiation from the laser diode bars is first emitted in a direction along the surface of the block, but is then reflected upward by angled reflectors on the block's surface.
A feature of laser diode arrays is the high intensity output provided from the closely packed laser diodes. Another advantage of diode arrays is that the output beam's area can be made larger simply by increasing the area of the array. To obtain the higher intensity, the laser diodes in the array should be positioned closer together. However, as a result of close positioning, the heat flux from each laser bar will add with the heat flux of the adjoining laser bar, and without aggressive cooling the temperature may increase rapidly. At a high output power (a high intensity and long duty cycle), the amount of heat flux produced in each diode becomes very substantial.
As a result, the average output intensity of a diode array is substantially limited by its ability to sink heat. Using only ambient air cooling, average power output must be limited by maintaining the current and duty cycle at a level sufficient to prevent damaging temperature buildup. There is a tradeoff between output power and output pulse duration; a long duty cycle must be balanced by a small current, and conversely, a large current must be balanced by a short duty cycle. The heat flux in a diode array is substantial during a period of high output. Without additional cooling, a laser diode array operated at a high average power will produce a large heat flux which can cause a rapid temperature increase, leading to device failure and other temperature associated problems discussed above. Therefore, a higher intensity output will generally require a more effective cooling system.
Two performance figures of merit for cooling purposes are thermal resistance and temperature uniformity. Thermal resistance is defined as the temperature rise at the laser junction relative to the coolant inlet temperature, per watt/cm.sup.2 of heat load. The causes and effects of heat load have been discussed above.
Temperature uniformity is a measure of the maximum temperature variation across the surface where the heat is applied. Lack of temperature uniformity could be caused by coolant heating or variations in heat load or thermal resistance, For applications such as high power laser diode arrays, a heat sink should have figures for thermal resistance and temperature uniformity small enough to dissipate the large amount of heat generated by high average power operation. In these high power applications, if a heat sink were available that had low figures for thermal resistance and temperature uniformity, the cost per watt of output laser power would be reduced significantly.
Another problem that arises in high average power laser diode arrays is thermal expansion mismatch. A material's thermal expansion coefficient describes the extent of the material's expansion caused by a temperature change. If two materials that have a thermal expansion mismatch are bonded together, then a temperature change may result in a cracking of the structure of one material or the other, or it may result in a compromise of the bond between the two materials. Gallium Arsenide (GaAs) is a conventional material for laser diodes; its thermal expansion coefficient is different than that of, for example silicon (Si), and many other materials.
Problems with low temperature uniformity, thermal resistance, and thermal expansion mismatch exist with regard to the stacked arrays. To address these problems, it has been suggested that thermally conductive material having a similar expansion coefficient be placed between the diode bars, in a "rack and stack" architecture. The thermally conductive material may be copper or a copper-tungsten alloy, such as Thermkon.RTM., a General Electric product, that is matched to the thermal expansion constant of GaAs. This material may be made thick to absorb excess heat. However, the thermal conductance of copper or Thermkon.RTM. is insufficient for operation at a high duty cycle. Another product used in the rack and stack architecture is beryllium oxide (BeO), which has an even lower thermal conductance, and therefore allows an even smaller duty cycle. BeO is used because its thermal expansion matches the thermal expansion of GaAs, therefore it provides a structurally sound package even while operating at high temperatures. It has also been suggested that a diamond material be used in the rack and stack architecture. Diamond has a high thermal conductivity; however, manufacturing is difficult and costly due to the diamond's hardness.
It would be an advantage if a laser diode could be cooled sufficiently to produce a high average power. It would be a further advantage if the laser diode and its cooling means were provided in a thin, easily constructed package that can be closely stacked in an array of laser diodes. Such a laser diode array would have many uses including application as an efficient pump source for pulsed solid state lasers.
Microchannel coolers (i.e., silicon wafers with small channels etched therein) have been used to cool integrated circuits, however they have not been applied in a practical way to cool laser diodes. One problem is the mismatch in thermal expansion between GaAs and silicon; the mismatch may lead one to the conclusion that a direct bond between the GaAs laser diode and the silicon wafers would lead to cracking of either the GaAs or the silicon, or failure of the bond between them. To avoid this problem, it has been suggested to use a material that matches the thermal expansion of GaAs, and insert that material between the GaAs and the silicon microchannel. A disadvantage of this approach is the increase in cost of manufacturing and the increase in size. The larger size increasing the area over which the laser's energy is spread, thereby decreasing output intensity.
Other problems in applying microchannel cooler technology to laser diodes include: delivering coolant to the microchannels proximate to the diode so that a high flow rate can be achieved, and mounting the diodes closely in an array to provide a high output intensity while still providing substantial cooling.
Conventional laser diode arrays have been formed of a number of laser diodes structurally connected together in such a manner that replacing a laser diode that is non-functional or otherwise not working is difficult or impossible. This disadvantage is particularly acute in manufacturing laser diode arrays; testing each of the laser diodes before installation is difficult. If it is later discovered that the one or more of the laser diodes does not function properly, then the whole array may have to be scrapped or remanufactured at a substantial cost. It would be an advantage to provide a package that has an edge emitting laser diode bar and a heat sink that can remove a significant amount of heat from the laser diode so that the laser diode can be operated at a high average power. It would be an advantage if the laser diode array could be formed of a number of similar laser diode and cooling packages that are interchangeable and easily replaceable, so that if a particular package becomes inoperable or of poor quality, then the package can be easily replaced at low cost without affecting the remainder of the diode array.